RATE OF WEARING OE COASTS 27 



They, in some way not yet well known, hut probiil.'K- hy the 

 rasping action of their shells, cut out little chambers in soft 

 rock which sometimes attain a depth of several inches. 

 Where these creatures are numc-rous they hone\coml) the 

 stone and make it so frail that the waves can break it up. 

 Certain of our echini, or sea-urchins, have in yet greater 

 measure this ability to bore into the rocks : they can by the 

 movement of their frail-looking spines tunnel downward in 

 materials as hard as orranite ; as their bodies are larger than 

 the lithodomi, they bore much greater holes. These cham- 

 bers are often as much as two inches in diameter with a 

 depth of a foot or more, and afford one of the most remark- 

 able evidences of the effort which organic forms make to avail 

 themselves of the profit which the shore conditions afford. 

 So far as has been observed, this habit of rock-boring on the 

 part of the sea-urchins is not known among our American 

 species, though it is common among their kindred ox\ the 

 shores of Europe. 



Hitherto we have been considering the action of the ocean 

 waves and currents on shores where the harder kinds of rock 

 meet the sea. Although this is the commoner condition of 

 the coast in its cliff-bordered sections, there are many steeps 

 formed by the frailer rocks, such as are afforded by the glacial 

 deposits of northern countries or the incoherent strata of the 

 newer geological formations, when the bits of such beds have 

 not been bound together in the firm way in which we find 

 them in most old deposits. Along the coast of the Atlantic, 

 from the mouth of the Hudson to Greenland, particularh" in 

 the southern portion of this shoreland. are hundreds of miles 

 of steeps where the sea beats directly against these yielding 

 materials ; operating on these clifTs the sea-waves dc^ not have 



